Observed correlation between frequency exponent and elevation angle of tropospheric scintillation

Scintillations are rapid fluctuations in amplitude and phase of the received signal arising from fluctuations in the atmospheric refractive index due to turbulence. Amplitude scintillation occurring in the troposphere increases with signal frequency and depends on a number of other link and meteorological parameters. In the design and link-budget calculations of low-margin VSATs and USATs communications systems scintillation effects need to be considered. In particular, knowledge of the frequency scaling of scintillation is important to the design and operation of fade countermeasure systems, such as uplink power control (ULPC). Scintillation effects are known to depend not only on meteorological parameters but also have systematic dependence, such as beacon frequency, antenna diameter and elevation angle. The analysis here is concerned with a new finding which suggest that a relationship exist between elevation angle and the frequency exponent α. In the existing semiempirical models α is constant. According to the Karasawa model [1988] it has a value of 0.45, based on measurements made during 1983 at Yamaguchi, Japan, at an elevation angle of 6.50, frequencies of 11.54 and 14.23 GHz, and an antenna diameter of 7.6 m. For the elevation angle dependence, they used long-term data from the same site at elevation angles of 40 and 90. The ITU-R Recommendation P618-7 contains another model, which has been validated using measurements covering elevation angles in the range of 4-320, antenna diameters between 3 and 36 m, a frequency range of 7-14 GHz. They adopted the theoretical value of 7/12, which comes directly from the theory.